This invention is related to the energy conversion in general and the utilization of thermal energy of the atmosphere, the ocean, a large lake or a large river for power production in particular. Waste water from industry and cities and other fluids of the so-called low quality thermal energy can also be utilized. The fluid or fluids discharged from these power plants are at very low temperature and therefore can be used for desalination of seawater and other cooling or refrigeration processes, such as the cooling of superconductors, air conditioning, etc.
It is known that the higher the source temperature and the lower the sink temperature, the higher will be the efficiency of thermal energy conversion.
The high temperature development has attained the allowable limit of material, whereas the lowest temperature has been limited by the ambient temperature of naturally available water and atmosphere. It has thus been concluded that available energy will become limited, as fossil and nuclear fission fuels become scarce, unless other heat sources can be found. The quantity of solar energy incident upon the earth is almost limitless, but it is diffuse. By using collectors, relfectors and absorbers, the energy thus produced is not only expensive but also unsteady. The atmosphere, the ocean, a large lake and a large river are natural solar-energy absorbers, but the utilization of ocean thermal energy has not developed beyond that of Claude-type power plants (Mechanical Engineering, Vol. 52, 1430) making use of the temperature gradient in a deep ocean with efficiency of only a few percent.
Further increase of efficiency requires a number of new concepts in thermodynamics and therefore, a brief discussion on the second law is in order.
(1) A heat reservoir is a concept representing a large body that remains at constant temperature regardless of the amount of heat transferred to or from it. Such a reservoir is a closed system as illustrated by FIG. 1 where W designates the work done by an engine and the temperature difference between the two heat reservoirs plays an important role in engine performance.
(2) The second law is the law of entropy which cannot decrease spontaneously or continuously in an isolated system.
(3) Kelvin and Planck stated that it is impossible to construct a device which operates in a cycle and produces useful work without other effects by taking heat from a single reservoir. There are many versions of the second law statement, but all are equivalent and based upon a closed system as shown in FIG. 1.
(4) The work produced by a cycle is limited by that of the Carnot cycle, but the work produced by a noncyclic process is not.
(5) The atmosphere or the ocean is often used to illustrate the conceptual heat reservoir and, according to the statement (2) above, it has been concluded that an engine cannot be built to produce continuously useful work by utilizing the amtosphere or the ocean as a single heat source. Therefore, thermal energy of environmental fluids in homogeneous condition has been considered as a dead state.
It is to be noted that a heat source or a heat sink can be either a closed or an open system while the conceptually defined heat reservoir on which the second law statement (3) is based is a closed system. The atmosphere or the ocean can be utilized as an open heat reservoir. An open heat reservoir is more versatile than a closed reservoir. For instance, an open-cycle engine is a system drawing heat from an open heat reservoir and discharging mass, heat and entropy to the environment of any temperature. In this case, the environment is just a dumping reservoir but is not the lower temperature reservoir defined conceptually in classical thermodynamics as shown by FIG. 1.
All versions of statement like (3) of the second law are made on the basis of a closed system enveloped by the surroundings which is an isolated system of indefinite extent. Now, if the enclosed system operates on a cycle only and exchanges heat with the surroundings as a single reservoir, then EQU dW=dQ
where W and Q denote the work done by and heat supplied to the closed system. If dW&gt;0 then dQ&gt;0 and the surroundings is cooled continuously. Hence entropy decreases continuously in the surroundings (an isolated system) and is in violation of statement (2). However, if the enclosed system involves both cyclic and non-cyclic operations, then EQU dQ=dW-dE (2.1)
where dE represents the change of total energy of the open system. For this case, the possibility that dW&gt;0, dQ&gt;0 and dS.gtoreq.0, where S designates the entropy, cannot be ruled out. If the non-cyclic operation can do work, and the cyclic operation is reversible, then the system can produce more work than the Carnot cycle operating between the same pair of heat reservoirs.
A substance is thermally at dead state only when its temperature is at absolute zero. However, if the heat sink is to be in fluid state, then its triple point may be considered as the dead state.